1. Lec 4: while loop and do-while loop

2. Review from last week if Statements if (num < 0.5){
...println("heads"); }
if –else Statements
if (num < 0.5){
...println("heads"); }
else {
...println("tails");

3. Review from last week if –else if Statements if (n < 0.4){
...println("heads"); }
else if (n <0.8){
...println("tails");
else if (n < 0.85){
...println("edge");
else {
... println("fell off table");

4. Review from last week How to store text data in a variable?
make String variable
String message = new String("Hello world");
System.out.println( message );
How to read input from keyboard?

5. Use a Scanner object! Scanner input = new Scanner(System.in);
like a vending
machine w/
three buttons
next()
nextInt()
nextDouble()
input

6. Scanner options Create a Scanner object and connect to keyboard
Scanner input = new Scanner(System.in); // make it
only make one Scanner object in your program
near top of program, before you read anything
a Scanner reads Strings using the next method
String word = input.next(); // read a String
a Scanner can also read int or double:
int age = input.nextInt(); // read an int
double value = input.nextDouble(); // read a double

7. while loops
A while loop is a way of repeating a set of instructions as long as some condition is met
In general, a while loop looks like this:
while(condition) {         statements }
The condition must be a boolean expression
There can be any number of statements inside the loop
Sort of like ``an if statement that keeps repeating until the condition becomes false''

8. A while loop example
int i = 0; while(i < 5) { System.out.println(i); i = i + 1; } System.out.println(); System.out.println("i=" + i); i
(i<7)
Console

9. Flowchart of previous slide

10. Processing a while loop
Java does the following when it meets a while loop:
1) Check to see if the condition is true
2) If the condition was false, bail out of the loop, otherwise go to 3)
3) Execute the body of the while loop and go back to 1)

11. Another while loop example
int i = 4; while(i > 0){ System.out.println(i); i = i - 1; } i
(i>0)
Console

12. Some common loop errors
False from the get-go
int i = 0; while(i > 7) {     System.out.println(i);     i = i + 1; }
nothing is printed!

13. Never terminates
int i = 0; while(i < 7) { System.out.println(i); i = i - 1; } This is an "infinite loop" In BlueJ, R-click red run indicator, reset JVM

14. Another no-no loop
int i = 0; while(i < 7) ; { System.out.println(i); i = i + 1; } This is an "infinite loop" In BlueJ, R-click red run indicator, reset JVM

15. What will this loop output?
int i = 10; while(i < 30) {     System.out.println(i);     i = i + 5; } i
(i<30)
Console

16. Another shortcut Programmers are very lazy
Instead of i = i + 1;
we say i++;
instead of i = i – 1;
we say i --;
instead of i = i + 5;
we say i +=5;
You can experiment with these in Lab 4 if you like

17. do loop
A do loop is very much like a while loop, except that the condition is tested at the end of the loop instead of the beginning
Looks like this: do {         statements } while(condition);
The body of the loop is always executed the first time through, since the condition isn't checked until the end of the loop

18. How many times does the loop cycle?
int i = 0; while(i > 0) { i = i - 1; } System.out.println(i); // prints 0

19. Very Similar, only do-while
i=0; do { i = i - 1; }while(i > 0); System.out.println(i); // prints -1 do-while always executes at least once

20. Example: User Input loop
// Ask for age until a valid number is entered Scanner input = new Scanner(System.in); int age; do { System.out.println("What's your age?"); age = input.nextInt(); }while (age < 0 ); //loop back as long as input is bad System.out.println("Oh yeah? Well I'm " + (age+1) +"!"); }

21. Flowchart of previous slide

22. Lab 4 Dr. Kow's Hamburger Palace
Use loops to draw text graphic (while)
read input and validate (do-while)
use a Scanner to read input
lettuce = input.nextInt();